U.S. Pat. No. 2,689,017 discloses a rainwater surface drain for use in roof drains of the kind incorporating a substantially cup shaped reservatory for water to be drained. The reservatory is connected at a center discharge outlet on the bottom side thereof with a drain outlet pipe and is covered with an upper strainer or grating member downstream of which insert means are arranged within the reservatory for breaking up of air that is entrapped with the water and for preventing formation of an eddy. These eddy preventing means comprise transversely extending cross baffles extending upwardly into said upper strainer and terminating in a spaced proximate relation to the bottom of the reservatory. The upper strainer of this known rainwater surface drain is either formed as a flat grate member or as a substantially dome shaped member which with an alternative design could be additionally provided at an upper position of the flat grate member for preventing debris from reaching the grating slots of the flat grate member that is positioned underneath thereof.
It is a general and quite well-known problem with such rainwater surface drains that the water entering the reservatory via the upper strainer forms a turbulence whereby any entrapped air greatly impedes the further flow of water towards the interconnected drain outlet pipe. As a result thereof water will therefore be backed up on the surface which is to be drained so that a flooding will occur.
With the arrangement of such eddy preventing means as transversely extending cross baffles no optimum solution for this particular problem has been reached, however, so that with different designs of such eddy preventing insert means trials have been made for improving the intake of water into the interconnected drain outlet pipe by substantially preventing a vortexing of the water within the reservatory in combination with a substantial prevention of air from entering into the drain outlet pipe.
Different designs have been introduced as so-called syphonic rainwater drain systems as for example disclosed in GB Patents No. 2 269 402, 2 285 460 and 2 321 067. These further known designs comprise in general a more or less funnel shaped reservatory accommodating an insert member in general in the form of a pointed cone the apex of which is directed towards the center of the inflow orifice of the drain outlet pipe. The pointed cone could also act when cooperating as a baffle means in cooperation with separate vane-like limbs which will provide distinct flow passages within the space between the pointed cone and the surrounding inner wall of the reservatory.
A further example is disclosed in the German Utility Model Registration No. 202 05 749.6 and comprises a cone shaped insert member which at a flat base surface in parallel and in spaced relation to an upper flat grate member starts with a slightly concave curvature that continues with a slightly convex curvature towards the apex of the cone. The surrounding inner wall of the reservatory is formed with corresponding complementary wall portions for providing an annular flow passage that establishes as well a syphonic effect. Such substantially cone shaped insert members result in general in an efficiency which under German standard conditions will be acceptable for drain outlet pipes with a standard nominal diameter of DN70. However the efficiency of such cone shaped insert members is not at all acceptable for drain outlet pipes having a larger nominal diameter of for example DN80 since in this case an increased amount of water is being backed up on the surface to be drained. A non-allowable flooding will therefore occur due to an impediment of the flow of water to the discharge outlet on the bottom of the reservatory.